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Motivation
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<H2 CLASS="section"><A NAME="htoc179">13.1</A>&nbsp;&nbsp;Motivation</H2>
Constraint logic programming uses logical variables. This means that
when a variable is instantiated, its value must satisfy all the
constraints on the variable. For example if the program includes the
constraint <I>X</I>&gt;=2, then any attempt to instantiate <I>X</I> to a value less
than 2 will fail.<BR>
<BR>
However, there are various contexts and methods in which it is useful
to associate (temporarily) a value with a variable that does not
satisfy all the constraints on the variable.
Generally this is true of <TT>repair</TT> techniques.
These methods start with a
complete, infeasible, assignment of values to variables and
change the values of the variables until a feasible assignment is
found.<BR>
<BR>
Repair methods are useful in the case where a problem has been solved,
but subsequently external changes to the problem render the solution
infeasible. This is the normal situation in scheduling applications,
where machines and vehicles break down, and tasks are delayed.<BR>
<BR>
Repair methods are also useful for solving problems which can be
broken down into quasi-independent simpler subproblems. Solutions
to the subproblems which are useful for solving the complete problem, 
may not be fully compatible with each other, or even completely
feasible with respect to the full problem.<BR>
<BR>
Finally there are techniques such as conflict minimisation which seek
solutions 
that minimise infeasibility.
These techniques can be treated as optimisation algorithms, whose
constraints are wrapped into the optimisation function. 
However they can also be treated as repair problems, which
means that the 
constraints can propagate actively during problem solving.<BR>
<BR>

	<BLOCKQUOTE CLASS="figure"><DIV CLASS="center"><HR WIDTH="80%" SIZE=2></DIV>
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	Repair is used for:
<UL CLASS="itemize"><LI CLASS="li-itemize">
Re-solving problems which have been modified
<LI CLASS="li-itemize">Combining subproblem solutions and algorithms
<LI CLASS="li-itemize">Implementing local search
<LI CLASS="li-itemize">Implementing powerful search heuristics
</UL>

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<BR>
<DIV CLASS="center">Figure 13.1: Uses of Repair</DIV><BR>
<BR>

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